Dual mode radar beacon antenna



Sept. 7, 1965 c. H. CHILD 3,205,498

DUAL MODE RADAR BEACON ANTENNA Filed Nov. 50, 1960 2 Sheets-Sheet 1INVENTOR. CLAUDE H. CHILD ATTOR NEY FIG. 4C

Sept. 7, 1965 c. H. CHILD DUAL MODE RADAR BEACON ANTENNA 2 Sheets-Sheet2 Filed Nov. 50. 1960 FIG. 7b

INVENTOR. H. CHILD CLAUDE FIG.

United States Patent 3,205,498 DUAL MODE RADAR BEACON ANTENNA Claude H.Child, Paramount, Califi, assigner to North American Aviation, Inc.Filed Nov. 30, 1960, Ser. No. 72,821 Claims. ((Il. 343-705) Thisinvention relates to an airborne antenna structure and particularly toan antenna radiating linearly polarized waves in a substantiallyhemispherical pattern.

This application is a continuation in part of my application SerialNumber 831,843 for a Dual Mode Radar Beacon Antenna filed August 5,1959, now abandoned.

The invention provides a flush-mounted antenna for airborne use capableof establishing a substantially hemispherical radiation patternespecially suited to use with a beacon. In the past, such radiationpattern coverage has been obtainable only by the simultaneous use of twoor more separate antennas, each of which provides a part of the desiredpattern. Such installations suffer from the added weight and complexityof operation imposed by multiple elements and the feed means associatedtherewith.

A radio beacon of the type herein described may be used in identifyingand tracking the separate planes which make up a squadron. Each of theplanes may be assigned an individual code designation, and by the use ofan antenna with a hemispherical radiation pattern may be located andtracked, regardless of its direction in azimuth from a monitoring orcontrol station. The invention may be extended in usefulness bydirecting one antenna downwardly from the under side of a plane andanother upwardly from the top side of the fuselage. In this way contactmay be maintained regardless of the attitude of the tracked plane. Itwill be obvious that the tracked plane may return an indication of itswhereabouts at its own volition or as an automatic response to a codedinquiry signal, and both techniques are well-known to those skilled inthe art.

The antenna of the present invention provides the desired radiationfield by utilizing two modes of propagation in a cylindrical waveguideconnected by a suitable matching impedance to one of the broad sides ofa rectangular guide. This permits substantial reductions in weight andsystem complexity, since it is only necessary for the transmitter tofeed into the rectangular guide.

It will be appreciated that the design of antennas for modern aircrafttravelling at supersonic speeds presents problems that diifer by anorder of magnitude from those encountered with aircraft travelling atsubsonic speeds. Any antenna projecting from the surface of the aircraftwill introduce problems of wind resistance or air drag of a very seriousnature, and an ideal antenna would be one which presented no obstructionto the flow of air over the airframe or wing surface. This inventionpermits the antenna to be mounted inside the fuselage, and tocommunicate with the outside atmosphere by means of a window imperviousto the passage of air but completely transparent so far as the passageof electromagnetic radiation is concerned. It has been my discovery thatby combining a first symmetrical radially polarized mode and a second T5mode in a coaxial line or in a section of circular or cylindricalwaveguide, the outer or terminal end of which may then be mounted flushwith the surface of the fuselage, such that the ratio of the peak fieldamplitude of the first mode to that of the second mode is substantiallydb, it is possible to produce a single antenna structure having asubstantially hemispherical radiation pattern, yet free from thenecessity for multiple antenna elements and feeds.

The terms circular waveguide and cylindrical waveguide may be usedinterchangeably. While these sections are physically cylindrical, thefields therein are treated as they exist in a plane perpendicular to thecylindrical axis. The intersection of the plane and the cylinder, ofcourse, defines a circle.

The objects of the invention thus include providing an improved antennaarrangement for airborne beacon equipment.

Another object is to provide a hemispherical radiation pattern forairborne beacon use.

A still further object is to provide for a hemispherical radiationpattern with a flush mounted antenna wholly contained within thefuselage of an aircraft vehicle.

Yet another object is to reduce the complexity and weight of antennasystems for producing a substantially hemispherical radiation patternwith a minimal antenna structure.

A further object is to reduce the weight and complexity of antennasystems where it is desired to obtain linearly polarized radiation inhemispherical patterns extending both upwardly and downwardly from anairborne fuselage.

These and other objects of this invention will become apparent from thefollowing specifications when taken with the accompanying drawings, inwhich:

FIG. 1 is a perspective view of a preferred embodiment of the waveguidestructure of the invention;

FIG. 2a illustrates the radial electric field which may be obtained byusing the circular or cylindrical waveguide portion of FIG. 1, actingalone;

FIG. 2b is a side elevational view of such a cylindrical waveguide,together with the electric field pattern produced thereby when acting inthe TM mode;

FIG. 3a is a representation of the electric field produced in thecylindrical guide portion of the antenna shown in FIG. 1 when operatingalone in the TE mode;

FIG. 3b illustrates the radiation field produced by the electric fieldin the antenna of FIG. 3a when operating in the T E mode, taken in theplane indicated by lines 8-8 of that figure;

FIG. 30 is a view corresponding to FIG. 3b but taken as indicated bylines CC of FIG. 3a;

FIG. 4a shows the electric field pattern obtained with the antenna ofFIG. 1 when operating with both the TM and the TE modes present;

FIG. 4b shows a quarter wave plate used to split the TE mode into twoparts, as shown in FIG. 40 vectorially;

FIG. 4c shows vectorially the component into which the T15 mode is splitby the quarter wave plate shown in FIG. 4b;

FIG. 5 shows in sectional schematic form the details of a section usedto match the impedance of the cylindrical waveguide section to that ofthe rectangular waveguide section;

FIG. 6 shows an alternative embodiment utilizing a coaxial line as theradiating. element;

FIG. 7a shows another preferred embodiment, utilizing coaxial elements,in the side sectional view;

FIG. 7b shows a sectional view, taken as indicated by line B-B of FIG.7a of the embodiment there illustrated; and

FIG. 8 is a schematic showing of the angular relations at the radiatingend of the antenna.

Referring now to the drawings for a more detailed understanding of theinvention, the improved antenna structure is shown in perspective inFIG. 1 separated from adjacent structure, and as it relates to thefuselage of an airborne vehicle in which it may be installed in thesectional showing of FIG. 5.

The operation will be described in terms of an antenna for transmittingpurposes, but it will be recognized that the receiving function may beobtained from the principle of reciprocity. Energy is fed from atransmitter, not shown, into the input, or open, end 1 of a rectangularWaveguide 2 and then is propagated through a transition section 4 into acircular or cylindrical waveguide section 5. The TE mode is the lowestorder mode of the rectangular wave guide shown, and is used to excitethe desired modes in the cylindrical section. Otherwise the use ofhigher order modes in the rectangular section would excite other andundesired modes in the circular section. The cylindrical section 5 isdisposed at right angles to the longitudinal axis of the rectangularguide section 2, and adjacent the end of that section opposite openend 1. The transition section 4 for matching the impedances of the twoguide sections may be conventional. One satisfactory form is that shownby FIG. 6.55, page 367, of Microwave Transmission Circuits by Ragan,published in 1948 by McGraw-Hill as Volume 9 of the Radiation LaboratorySeries. While the effect of such transition section is to provide animpedance match or maximum power transfer from the rectangular section(as a power source) to the cylindrical section (as a load impedance)such transition section effects a change in mode of the energytransferred from the TE mode to the TM mode. Further, such transitionsection effects a discontinuity in the cylindrical section to the extentthat the TE mode is thereby excited. The relative magnitude of the TEmode field peak to the TM field peak increases as the discontinuityeifect presented by the transition section increases. Therefore, onemeans for controlling the several mode magnitudes so as to obtain thecritical ratio of substantially 10 db is the varying of the TM purity ofthe transition section. Such mode purity is defined on page 99 ofMicrowave Transmission Circuits (Volume 9 of Radiation LaboratorySeries, McGraw-Hill (1948)) as the extent to which the transition fromrectangular waveguide section operating in the lowest or TE mode to around waveguide operating in the second or TM mode occurs withoutexcitation of the lowest or TE mode. For example, in order to increasethe relative magnitude of the TE mode one would increase thediscontinuity afforded by the transition section. Limitations on thefeasibility of such a means is its reliance on empirical procedures inmanufacture of such means, and the difiiculty in achieving a closetolerance on the critical mode magnitude ratio.

There are present in the cylindrical waveguide waves whose radialcomponents of electric field may be represented by the equation for theTE mode:

E.=Eh sin e (wt (1 11 and for the TM mode, which may be represented by:E.=E.e (wtwhere E =the amplitude of the TE mode wave E zthe amplitude ofthe TM mode, Wave X =the distance along the axis of the cylindricalguide 4) & 9=the standard IRE field angles, as shown in FIG- 8 x =theguide wavelength of the TE mode wave x =the guide wavelength of the TMmode Assuming matched conditions for both modes, the total radialelectric field is given by where a is the phase difierence between themodes when X =0 and t=0. Dividing the righthand side of Equation 3 byand introducing as a new factor I i i k 7 11 M1 Equation 3 may bereduced to the form The equations set forth above indicate that the TMmode has no dependence on 41 and hence will give rise to a radiationpattern similar to that of a monopole over a ground plane, namely asemi-toroid. The dependence of the TE mode on causes a radiation patternwhich varies as a function of 1 The radiation patterns generated bythese modes in the planes AA and B-B of FIG. 1 are shown in the severalparts of FIGS. 2 and 3, and the angular relations are shown in FIG. 8.FIG. 2b shows section A-A, which is typical of all sections rotatedabout the Z axis for the TM mode alone. The pattern is shown in FIGS. 3band 3c for the TE mode alone: FIG. 3b showing the pattern as taken alongaxis BB of FIG. 3a; and FIG. 30 showing that taken along axis C-C. Whenboth modes TM and TE are present, the radiation pattern will be thevector sum of these individual patterns. The combined result, if themodes are present in the proper relative amplitudes of E and E is thatshown in FIG. 4a. The proper relative amplitude of E to E, is the ratio10 db. In connection with those transition sections which provide bothTM and TE modes having somewhat like amplitudes, this ratio is readilyobtained by the use of a mode filter 12 (in FIG. 4b) which passes the TMmode substantially unattenuated and which serves to attenuate the TEmode to the desired amplitude ratio. Such a device may be conventionaland is well understood in the.

art. For instance, such device may be of the type more particularlydescribed on pages 391-393 of Microwave Transmission Circuits (Volume 9,Radiation Laboratory Series, McGraw-Hill 1948) The waveguide structureof FIG. 1 may be mounted in the fuselage 6 of the ship, as shown in FIG.5, with the axis of the cylindrical portion vertical to the horizontalfor level flight, in order to radiate the pattern of FIG. 4aomni-directionally upward. The guide is secured by means such as aflange 7 to the fuselage by means of suitable fasteners, such as bolts8. The cylindrical section 5 is sealed to prevent passage of air betweenthe inside and outside of the aircraft by means such as a window 9,impervious to air but transmitting electromagnetic radiations freely.Polyethylene is a suitable material for such a window, but anyequivalent may be substituted.

Because the radiation pattern produced by this antenna is a function ofthe radial electric field in a cylindrical guide, it will be apparentthat the construction is not limited to a hollow waveguide as described,but may also take any form capable of producing the desired radialelectric field. For example, the combination of the TEM and TE modes ina coaxial line will produce such a field and, hence, the desiredradiation pattern. This has been illustrated in FIG. 6 in schematicform, with arrows indicating the direction of the radial field for theTEM mode and the field for the TE mode, corresponding to the showing inFIG. 3a.

Another coaxial line embodiment is shown in FIG. 7a in side sectionalview, with a transverse section taken along a line BB of that figureillustrated in FIG. 7b.

Here a coaxial line 30 having a center conductor 31, and of proper sizeto support the TEM mode only, is joined by a tapered transition section32 to a section 34 of diameter sufiicient to support both TEM and TEmodes. The section 34 is surrounded by a correspondingly increaseddiameter outer conductor portion 36. The second mode, TE is excited byintroducing a cylindrical metal plug or screw element 35 into the spacebetween the inner section 34 and the outer conductor 36. The shortingeffect of the plug upon the field in that area creates the discontinuitywhich excites the TE mode. The ratio or relative magnitude of the TEmode peak vector thus propagated to that of the TEM mode peak vectorincreases as the diameter of the plug is made to increase, thence, thedesired db ratio of TEM mode field strength peak, E to TE mode fieldstrength peak, E may be obtained by adjusting such dimension until suchratio is obtained. The axial distance of the plug 35 from the taperedsection 32 is selected to aid the impedance matching function of section32. The outer end 37 is shown as broken away, but it is understood thatit may be terminated by a window such as that shown at 9 in FIG. 5, orthe window omitted, depending on whether or not a solid dielectric isused in the line between the inner and outer conductors, as explained inconnection with the figure.

Assuming an upright orientation of the configuration of FIG. 1, thefield pattern in the ground plane (0:90 degrees), will not be entirelycircular (not radially uniform for all values of p or azimuth), due tothe difference in field strength between the TE mode plane of symmetryand the transverse plane normal to the plane of symmetry, as shown inFIGS. 3b and 3c. Further, such patterns would be slightly skewedrelative to the waveguide axis in the azimuth direction of the TE modeplane of symmetry due to the TE mode polarization. However, the skewedeffect is slight due to the low amplitude ratio of the TE mode to the TMmode.

Both these effects (1) failure of uniform field strength in azimuth, and(2) skew or radiation pattern axis inclination, can be offset or avoidedby resolving the TE mode vector into two mutually orthogonal vectors ofsomewhat similar amplitudes, and if further refinement is desired, byimparting an angular velocity to the TB mode about the waveguide axis ofRF magnitudes. This refinement to the embodiment of FIG. 1 is depictedin FIG. 4b, in which a dielectric quarter wave length plate 13, having awidth equal to the diameter of the waveguide section is contained withinthe cylindrical section 5 at the aperture end thereof and orientedradially about the waveguide axis of the cylindrical section as to forman angle of substantially 45 degrees with the waveguide axis of therectangular waveguide section. This orientation thus provides a 45degree angle between the quarter Wave plate and the incident plane ofsymmetry of the incident TE mode Wave travelling from the transitionsection. The TE mode vector is thus split into two quadrature componentshaving differing phase velocities, one component parallel to the plateand having a greater phase velocity than that of the other componentwhich is perpendicular to the plane of the plate. The effect of thediffering phase velocities is (1) a separation of the TE mode componentsin time and space at the end of the plate as shown vectorially in FIG.40 and (2) a rotational velocity of the resultant vector about thecylindrical waveguide axis at the aperture. The quarterwave plate 13 hasno effect upon the TM mode. The resultant of the combined TM mode androtating vector of the resolved TE mode is a radiation pattern similarto that in FIG. 4a for all sections containing the Z axis or waveguideaxis. For the coaxial section embodiment shown in FIGS. 7a and 7b, arefinement in structure analogous to the quarter wavelength plate wouldbe the addition of a second plug (not shown) oriented radially at 90degrees to the first plug 35.

A specific embodiment of this antenna which has been utilized fortesting autonavigating equipment had the following equations andconstants applicable: in the cylindrical waveguide, assuming that theradius a was equal to 3.80 centimeters, then the cut-01f wavelength, Ac,for each mode may be determined as follows:

where U' =nth root of the Bessel function J' (U)=0 Then for the TE modeA =12.95 centimeter for For the TM mode:

A =9.92 centimeters where U =2.405

The cut-off wave length so computed for each mode may be shown to belong enough as to indicate that the cylindrical section selected willsupport the two modes sought to be employed, in accordance with formulaswell known to those versed in the art.

Thus, by a combination of a first symmetrically radially polarized fieldand a second T E mode field in the cylindrical waveguide, the relativemagnitude of the first field to the second field being the ratio of 10db, the energy introduced into the waveguide can be made to radiate fromthe free end thereof in a substantially hemispherical pattern. Inneither of the embodiments shown is the length of the output waveguidesection critical.

When applied to an airborne device such as the monitoring or controlplane, a pair of such antennas, one mounted to radiate hemisphericallyupward and one mounted to radiate hemispherically downward, providesubstantially complete omni-directional covering for radar beacon use.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by Way of illustration andexample only and is not to be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaims.

I claim:

1. In an antenna system the combination of a rectangular waveguidesection; a cylindrical waveguide section for supporting a TM mode andcommunicating with a broad side of said rectangular guide section;impedance matching means disposed between said rectangular and saidcylindrical sections and presenting a discontinuity for exciting a TEmode, the relative magnitude of the TM mode to the T5 mode being 10 db;a quarter wave plate disposed in said cylindrical section symmetricallyabout the longitudinal axis of said cylindrical section at an angle of45 degrees with the waveguide axis of said rectangular waveguidesection; and means for mounting said system in an outer wall of afuselage, said means being transparent to electromagnetic radiation andadapted to close the end of said cylindrical section opposite to thatcommunicating with said rectangular section.

2. In an antenna system for use in an airborne radar beacon device, thecombination of a rectangular waveguide section; a cylindrical waveguidesection capable of supporting both a TM mode and TE mode and of diametersubstantially similar to the width of a broad side of said rectangularguide section, and communicating therewith; said cylindrical sectionhaving its axis of revolution disposed substantially normal to thelongitudinal axis of said rectangular section; a transition disposedbetween said rectangular and said cylindrical waveguide sections, andarranged to match the impedance of said cylindrical section to that ofsaid rectangular section and to excite a TE mode; mode filter means formaintaining the relative magnitude of the TM mode to the TE mode in theratio 10 db; a quarter wave plate disposed in said cylindrical sectionsymmetrically about the longitudinal axis of said cylindrical section atan angle of 45 degrees to said longitudinal axis of said rectangularsection; and means for closing the end of said cylindrical guide sectionopposite that associated with said rectangular section to the passage ofair while permitting unobstructed passage therethrough ofelectromagnetic waves.

3. In an antenna system for use in an airborne device, the combinationof a rectangular waveguide section; a

cylindrical waveguide section communicating with a broad side of saidrectangular section; transition means disposed for matching theimpedance of said sections associated therewith; means for setting upelectric fields in said cylindrical guide in the TE mode; means forsimultaneously therewith setting up electric fields in said cylindricalguide in the TM mode; the relative magnitude of the TM mode peak vectorto that of the TE mode being the ratio 10 db; means for splitting thefields in the T13 mode into two quadrature components of substantiallyequal amplitudes in said cylindrical guide; and means transparent toelectromagnetic radiation for closing off the end of said cylindricalsection opposite said rectangular section from the exterior of saidairborne device, whereby the resultant radiation pattern issubstantially hemispherical about the axis of revolution of thecylindrical Waveguide section.

4. In an antenna system the combination of a rectangular waveguidesection; a cylindrical waveguide section capable of supporting both a TMand a TE mode, communicating with a broad side of said rectangular guidesection; means for producing said TM and said TE modes; means forcausing the relative magnitude of the TM mode to the TE mode to be 10db; and means for producing a rotational phase of said T mode relativeto the TM mode. 5. In an antenna system the combination of a rectangularwaveguide section; a cylindrical waveguide section, capable ofsupporting both a TM and a TE mode, and communicating with a broad sideof said rectangular waveguide section; impedance-matching means disposedbetween said rectangular and said cylindrical sections; means forproducing said TM and said TE modes; means for causing the relativemagnitude of the TM mode to the TE mode to be db; and means, comprisinga quarter wave plate disposed in said cylindrical Waveguide section atan angle of 45 10 degrees with the Waveguide axis of said rectangularwaveguide section, for producing a differential phase velocity of saidTE mode relative to said TM mode.

15 References Cited by the Examiner UNITED STATES PATENTS 2,519,750 8/50Ehlers .33321 2,761,138 8/56 Sherman 343783 X 2,774,067 12/56 Bollinger343783 X 2,816,271 12/57 Barker 333--21 X 2,881,432 4/59 Hatkin 343783 X2,939,094 5/60 Berk 333--21 OTHER REFERENCES Army Technical Manual, TM11666, Feb. 9, 1953, pp. 120, 121 and 122 relied on.

Microwave Transmission Circuits; Ragan, McGraw-Hill 1948 (Vol. 9, Rad.Lab. Series), page 367.

3 HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, ELI LIEBERMAN, Examiners.

1. IN AN ANTENNA SYSTEM THE COMBINATION OF A RECTANGULARR WAVEGUIDESECTION; A CYLINDRICAL WAVEGUIDE SECTION FOR SUPPORTING A TM01 MODE ANDCOMMUNICATING WITH A BROAD SIDE OF SAID RECTANGULAR GUIDE SECTIN;IMPEDANCE MATCHING MEANS DISPOSED BETWEEN SAID RECTANTULAR AND SAIDCYLINDRICAL SECTIONS AND PRESENTING A DISCONTINUITY FOR EXCITING A TE11MODE, THE RELATIVE MAGNITUDE OF THE TM01 MODE TO THE TE11 MODE BEING 10DB; A QUARTER WAVE PLATE DISPOSED IN SAID CYLINDRICAL SECTIONSYMMETRICALLY ABOUT THE LONGITUDINAL AXIS OF SAID CYLINDRICAL SECTION ATAN ANGLE OF 45 DEGREES WITH THE WAVEGUIDE AXIS OF SAID RECTANGULARWAVEGUIDE SECTION; AND MEANS FOR MOUNTING SAID SYSTEM IN AN OUTER WALLOF A FUSELAGE, SAID MEANS BEING TRANSPARENT TO ELECTROMAGNETIC RADIATIONAND ADAPTED TO CLOSE THE END OF SAID CYLINDRICAL SECTION OPPOSITE TOTHAT COMMUNICATING WITH SAID RECTANGULAR SECTION.